I made this Popsocket mount because I wanted to be able to hang up my phone in lots of places without having to spend $10 per mount! I've included the .blend file in case you want to add holes for screws, ziptie anchors, etc. Should be easy to modify. 

I made a video demonstrating how it works...

Print it with supports so that the overhanging part comes out OK. I printed it with a 0.2mm layer height with a 0.4mm nozzle on an Original Prusa i3 MK2 in eSun Blue PETG.

IMPORTANT NOTE FOR MOUNTING IN CARS: If you plan to use thing in your car make sure you print it in PETG, ABS, or other similar heat-resistant plastic. PLA will melt! I recommend at least 25% infill to make sure the part stays reasonably strong. It should print OK in PLA, PETG, ABS, etc. It doesn't need to flex (the Popsocket is what flexes). I made it to print flat but you can probably print it without (as much) supports if you print it upright.